Cyclo[18]carbon and Beyond: New Materials, New Properties, and New Opportunities

ConspectusCarbon-based allotropes enriched material theory and played an irreplaceable role in driving technological revolutions in the fields of energy conversion, communication sensing, and data storage. While enjoying the changes brought by known carbon materials, researchers are actively exploring the design of new carbon allotropes with excellent performance. Cyclic molecules based on sp-hybridized carbon, known as cyclocarbons, have fascinated chemists for many years, and a great number of experimental explorations on this two-coordinated carbon cluster have been conducted. Evidence of the existence of cyclocarbons in the gas phase has been repeatedly discovered, but for a long time, no one has been able to provide a visual image of their morphology. It is only recently that their first member, cyclo[18]carbon (C18), and subsequent homologues have definitely been generated and clearly characterized in the condensed phase. Unfortunately, nontraditional preparation methods and high coupling reactivity hinder experimental exploration of their properties and applications at present. In this context, theoretical computational simulations provide an alternative means for further studying them. Taking experimental imaging and structural analyses as reference, theoretical studies have made great progress in understanding the unique properties and predicting the potential applications of C18. In addition, related research on its analogues has also been widely reported. These studies have greatly promoted our understanding of C18 and its analogues. Therefore, collecting and introducing the latest progress in the theoretical investigation of C18 are of great significance for accelerating the development of carbon nanomaterials in general.In this Account, we first briefly reviewed the process of C18 from being noticed by people to finally being observed in an experiment. Then, after presenting the computational strategy for conducting credible theoretical research on C18, we summarize our recent contributions to the prediction of physicochemical properties of C18, with a focus on the electronic structure, aromatic character, optical property, and thermodynamic behavior. The relevant properties of the C18 analogues are subsequently generalized. At the end of this Account, regarding the current research progress and obstacles on cyclocarbons in experiments, the existing challenges and future perspectives for theoretical calculation and simulation of cyclocarbon systems are presented. It is proposed that there is an urgent need for theoretical research on the coupling mechanism of individual cyclocarbons and the properties of bulk materials in practical environments; for the latter it is necessary to develop suitable force fields for simulation studies. We believe that this Account will inspire innovative theoretical research in the field of cyclocarbons, especially by incorporating them into the research field of novel carbon materials.